A common type of interior decorative trim panel or cover used to cover and hide the air bag in a motor vehicle comprises a thin elastic plastic skin having a non-glaring grained outer surface, a rigid retainer or substrate and an intermediate layer of soft plastic foam between the skin and substrate. The present invention relates to those covers wherein the skin is formed of a thermoplastic polymer or thermosetting resin and is provided with a tear seam, and the substrate includes one or more doors that are impacted by the inflating air bag and press against the foam layer and the skin to tear away a counterpart portion of the foam layer while also tearing the tear seam and then swing outward with the torn foam layer and a counterpart torn portion of the skin to form an opening in the cover for deployment of the air bag into a protective position in the passenger space. These tear seams are provided in various configurations or patterns with the most common having a C, H, U, or X-shape and wherein the pattern determines the number of doors required in the substrate.
It is desirable that the tear seam and thereby the presence of the air bag be hidden from view for various reasons and heretofore, this has been accomplished in several different ways. Such a tear seam is commonly referred to as an "invisible tear seam". One way of providing such an invisible tear seam is by forming a tear seam defining groove or series of depressions in the backside of the skin. This leaves a thin and thereby weakened section at the outer or appearance side of the skin that defines the tear seam without outwardly revealing its presence. Examples of such invisible air bag cover tear seams are disclosed in U.S. Pat. Nos. 5,072,967; 5,082,310; 5,316,822 and 5,348,339. The tear seam may be formed in the backside of the skin in various ways as is well known by those skilled in the art. For example, the tear seam may be formed in the backside of the skin with a so-called "hot knife" tool in the case of a powder cast or vacuum formed skin or with a tear seam forming feature in the mold tool for the skin's backside where the skin is compression or injection molded. Another way of forming an invisible air bag tear seam in the skin is to make the skin in a powder casting process and make the tear seam as a strip of a weaker like colored plastic material that is integrally joined with the skin in indistinguishable surface appearance relationship therewith as disclosed in U.S. Pat. No. 5,288,103. In the latter method, a tear seam defining gap is formed in the skin with a gasket that is pressed by a pattern against the mold surface of a heated shell tool while a thermoplastic powder is cast from a powder box against the mold surface and about the gasket to form the skin and this gap is then filled on the heated mold surface with the weaker plastic material to form the integral tear seam strip following removal of the gasket.